Prepared by. Drew Davidge

Transcription

1 Flood Damage Estimation in the Upper Thames River Watershed CFCAS project: Assessment of Water Resources Risk and Vulnerability to Changing Climatic Conditions Project Report VII. August 2005

2 Prepared by Mark Helsten and Drew Davidge CFCAS Project Team: University of Western Ontario Slobodan P. Simonovic Gordon McBean Predrag Prodanovic University of Waterloo Donald H. Burn Karen Wey Linda Mortsch Paul Kay Andrea Hebb Ainslee Emerson Upper Thames River Conservation Authority Rick Goldt Mark Helsten Drew Davidge - 2 -

3 Contents I. Introduction 4 II. Study Area 5 II.1 Flooding in the Upper Thames Basin 8 III. Previous Work Done 9 III.1 Flood Depth-Damage Estimation 10 IV. Flood Damage Estimation Methodology 11 IV.1 Hydrology 11 IV.2 Hydraulics 11 IV.3 Flood Damage Estimation 11 V. Flood Damage Estimation Modelling 13 VI. Using HEC-FDA to perform a Flood Damage Analysis 16 VI.1 Study Configuration 16 VI.2 Stream Delineation 16 VI.3 Damage Reach Delineation 16 VI.4 Hydrologic Engineering 17 VI.5 Economics 18 VI.5.1 Application of Depth Damage Tables 18 VII. Study Damage Reaches 20 VII.1 Mitchell 21 VII.2 St Marys 22 VII.3 London 23 VII.4 Ingersoll 24 VIII. HEC-FDA Results 25 VIII.1 Flow Damage Curves 25 VIII.1.1. Mitchell 26 VIII.1.2 St Marys 28 VIII.1.3 London 30 VIII.1.4 Ingersoll 37 VIII.2 Aggregated Flow Damage Curves 38 VIII.2.1 Mitchell 39 VIII.2.2 St Marys 39 VIII.2.3 London 40 VIII North Branch 40 VIII South Branch 41 VIII Main Branch 42 VIII.2.4 Ingersoll 42 IX. Conclusions 44 X. References 45 List of Figures Figure 1. Thames River Watershed Location 6 Figure 2. Watershed Map of the Upper Thames River Conservation Authority 7-3 -

4 I. INTRODUCTION This report presents the findings and methodology of the flood damage estimation completed in conjunction with the Canadian Foundation for Climatic and Atmospheric Sciences (CFCAS) study Assessment of Water Resources Risk and Vulnerability to Changing Climactic Conditions for the Upper Thames River Basin. One aspect of this study examines different climate change scenarios and associated increases in occurrences of flooding and associated damages

5 II. STUDY AREA The Upper Thames River Basin lies in the middle of South Western Ontario, and drains 3432 km 2 of area, and populated by approximately people. Land use in the watershed is about 80% Agricultural, 10% urban and small towns, and 10% forest cover. The Thames River is comprised of two branches which meet at a confluence in London (known locally as The Forks ). One branch drains the northern portion of the watershed (North Thames River, 1750 km 2 ) and the other drains the Southern portion of the watershed (Thames River, 1360 km 2, above the forks). Downstream of the forks, the river eventually exits the upper portion of the Thames River basin and enters the Lower, ultimately flowing into Lake St. Clair. Flows on the river are attenuated by 3 major flood control structures, one on Trout Creek (Wildwood Reservoir), a tributary of the North Branch, one on the North Thames River directly upstream of London (Fanshawe Reservoir) and one in the upper reach of the Thames River in Woodstock (Pittock Reservoir)

6 Figure 1. Thames River Watershed Location - 6 -

7 Figure 2. Watershed Map of the Upper Thames River Conservation Authority - 7 -

8 II.1 Flooding in the Upper Thames Basin The Upper Thames River basin has a long and well documented history of flooding, going as far back as the late 1700 s. Major flood damage centers in the watershed include: London St. Marys Ingersoll Mitchell Stratford Woodstock As the watershed is largely agricultural, there is also the potential for agricultural flood damages to occur. However, in reviewing the historical record of flooding available, little note has even been made of agricultural damages due to flooding in the watershed

9 III. PREVIOUS WORK DONE As the Upper Thames basin has been settled and populated for quite some time, there is a significant flood control infrastructure in the major damage centres which has been built over the years. These structures include the three flood control dams mentioned above, and also a series of dykes in London, and a flood wall in St. Marys. During the 1970 s and early 80 s, a feasibility study was completed for a proposed dam, which would create the Glengowan Reservoir, to be located on the North Thames River, upstream of St. Marys, partially to protect the downtown area of St. Marys from flooding. As part of this study, a report called the Hydrological and Flood Damage Study was produced, which created a HYMO model of the watershed, and, most importantly for the present study, did an extensive inventory of all structures in the floodplain downstream of its proposed location. This work included detailed level surveys of ground, first floor, lowest opening elevations and occupancy type for all properties in the floodplain at that time, Also included in the Glengowan report was a detailed review and analysis of generic flood depth-damage tables existent at that time. The surveyed data still exists in paper format, and one task of the present study was to digitise all of this information. The Glengowan Reservoir was eventually deemed unfeasible, with the recommendation that a better cost-benefit ratio would result from constructing a floodwall in St. Marys, built to the elevation of the regulatory (1:250 year) flood. As part of the design of the St. Marys Floodwall, more detailed surveys were done for all structures within the downtown of St. Marys in the flood plain

10 A further flood control structure improvement made in the early 1990 s was the raising of the Broughdale dyke in on the North Thames River from its existing level to the regulatory flood level. Also as part of the background work for this project, detailed level surveys were done for all of the structures that it protects. A flood damage assessment inventory was done by the Upper Thames River Conservation Authority in 1998 in the town of Mitchell, without detailed level surveys, but with topographic maps and windshield surveys. III.1 Flood Depth-Damage Estimation There has been a significant amount of work done in the past in Canada to determine the damage resulting from different depths of flooding in a variety of types of structures. The Glengowan Hydrological and Flood Damage Study compiled a series of these Depth-Damage tables based on information from the US Flood Insurance Administration (FIA). A study was also completed by Paragon in 1984 for the Ontario Ministry of Natural Resources (MNR) on estimating flood damages in residential homes in Ontario. All of this work was summarised in the 1989 MNR document, Flood Damage Estimation Guide, and this is the source of depth-damage tables used in the present study

11 IV. FLOOD DAMAGE ESTIMATION METHODOLOGY IV.1 Hydrology The first step in completing a thorough flood damage estimate is the completion of a hydrological analysis of the area being studied. This can vary from simply using transposed flood frequency data, from locations where this is known, to a regional analysis, to the running of a fully calibrated hydrologic model. In the case of the Upper Thames basin, in the areas where flood damages are most severe, hydrologic analysis has been undertaken extensively, including both flood frequency analysis of the stream stations in the watershed, and detailed modelling done for the Glengowan study as well as the various subwatershed studies completed on major Thames river tributaries. IV.2 Hydraulics Once the hydrologic analysis is completed, this data is used in some form of hydraulic or backwater model to estimate what elevation the stream will rise to for a given series of flows at various reaches throughout the river. As with hydrologic analysis in the Thames basin, hydraulic models, originally coded in HEC-2 and since translated to HEC-RAS, have been completed in all flood damage centres in the watershed for the Thames River at its major tributaries. Much of the hydraulic and hydrologic modelling was done as part of the Federal Flood Damage Reduction Program (FDRP) in the 1980 s IV.3 Flood Damage Estimation Techniques for estimating flood damages can range from a cursory survey using only topographic maps, to detailed surveys of each individual structure. As mentioned earlier, the Upper Thames basin has had some work done in terms of detailed structure

12 surveys, in the damage centres of St. Marys and London. However the other major damage centers had either no work done on their structure inventories, or a cursory analysis done in the case of Mitchell. The most significant damage center undocumented was the Town of Ingersoll. In this case, structures within a potential floodplain were identified using a GIS based technique. The existing 250 year (regulatory) flood line was superimposed on a map of the town, and any structures within that zone, plus any within a buffer beyond were flagged (meant to approximate the 1:500 year flood). An automated routine was then used which examined each structure in turn, and using a digital elevation model of the town, the lowest ground elevation for each building footprint was estimated. This information, including building area, was recorded into a database for future use. Several trips were also made to the town to estimate and record the distance of the lowest opening above the ground, as well as they occupancy type for commercial structures. Trips were also made to the damage centres in St. Marys and London to verify the previously recorded information, and to update occupancy types for commercial structures. When superimposing the approximate 1:500 year flood line on the Cities of Woodstock and Stratford, few structures were identified as flood-prone

13 V. FLOOD DAMAGE ESTIMATION MODELLING In order to estimate the amount of damage caused by a given severity of flood, all of the information compiled in the above processes must be taken into consideration. Currently there are a few models available to accomplish this task: FLDDAM was a program written for the MNR in conjunction with their Flood Damage Estimation Guide in While the program is still available, we felt that we should investigate what other products are available. Following is a brief review of available flood damage estimation models. HAZUS-MH is a multi-hazard damage estimation model produced by the US Federal Emergency Management Agency (FEMA) for estimating potential losses from earthquake, wind and flood disasters. It is GIS based, and appears to be an excellent tool for damage estimation, but it seems to have been created largely for the US insurance industry and it is unclear if this could be adapted for use in Canada, or if it is even available outside the US. URB1, ECON2 are flood damage estimation models produced by the US Department of Agriculture (USDA) for estimating both urban and agricultural flood damage respectively. Like the MNR s FLDDAM model these programs are DOS-based with data entry requiring a separate program. While these programs are still available for free, if we are going to replace the FLDDAM package, we hoped for a newer package which was more user friendly, in a windows format. FloodEcon is also mentioned by the USDA as a newer update, combining URB1 and ECON2. However, their website indicates that work was being done in 2003, but no more recent mention was made of it. Also there was indication that the USDA is moving

14 towards converting to the USACE HEC-FDA damage assessment program (see below) rather than focusing efforts to update their own models. DAMS/DAMP was a combination package for both archiving stream gauge data from remote sites and assessing potential flood damages produced under the supervision of Conservation Halton in the 1990 s. This program appeared to have good potential when the UTRCA reviewed it in the late 90 s, however it had some bugs in it, and was never taken to a final stage, so unfortunately we did not consider it as a potential model. HEC-FDA is the flood damage estimation model produced by the US Army Corps of Engineers (USACE). It is a risk-based analysis tool intended for use in the feasibility analysis phase of different flood mitigation measures, including a without project scenario. HEC-FDA has a function to import HEC-RAS and HEC-2 files (provided those packages are configured to produce output in the FDA format), and runs in a windows environment. HEC-FDA is a free piece of software, and includes extensive documentation. Upon reviewing the various models available for performing flood damage estimation, it was pretty clear that HEC-FDA offered the best alternative due to it current status and availability (now running version 1.2, with version 2.0 to be released soon), connection to existing HEC-RAS model output, windows running environment, ease of use and extensive documentation. All of the data collected in the past and for the current study, including structure information, HEC-RAS flood line output, and generic commercial and residential depth

15 damage curves were thus entered or imported into the HEC-FDA model and reach specific depth damage curves created

16 VI. USING HEC-FDA TO PERFORM A FLOOD DAMAGE ANALYSIS VI.1 Study Configuration The first step in setting up a HEC-FDA analysis is study configuration, which for the purposes of this study includes the delineation of streams and damage reaches. For a more involved feasibility study, the configuration would also include different plan information and a summary of analysis years. VI.2 Stream Delineation Stream Delineation is straight forward for this project, with each stream in the particular study area being named, described and added as required. VI.3 Damage Reach Delineation Damage reaches are set up in spatial floodplain areas, and are delineated by beginning and end reaches. Generally all structures, damage reaches, index locations, HEC-RAS cross sections etc. are all specified by a chainage distance, usually from the downstream end of the study. Any number of damage reaches can be set up along a stream, and they can be specified for either the right or left overbank (looking downstream), or for both. Damage reaches are integral to both the economic and hydrologic analyses, and are generally set up to coincide with the location or potential location of flood damage reduction measures. They must also be consistent with the flood damage reduction measure they are modelling, for example a channelisation project would be analysed for both overbanks, and a levee or floodwall for only the bank it is protecting. Damage reaches are not easily changeable once the project is created,

17 so a fair bit of thought should be put into this phase of the study configuration. For the City of London and the Town of St. Marys damage reaches were adopted from past studies, with only minor variations. When creating a flood damage reach, one must specify upstream and downstream locations (using the same chainage system as the HEC-RAS model output), and also an index location. The index station is where stage-damage functions are aggregated for the damage reach, and may be located anywhere within the damage reach. If there is a stream gauge present in the damage reach, this is the most sensible place to use for the index location, otherwise judgment must be made as to use the upstream, downstream or middle of the reach. VI.4 Hydrologic Engineering The Hydrologic Engineering portion of the data is where water surface profiles, hydrology and flood control structure data (ie dykes, levees) are input. Data from the HEC-RAS model of the damage centre under study is exported to a format which HECFDA can read. The cross sections in the UTRCA HEC-RAS models were renumbered such that they corresponded to chainage distance from the forks of the Thames. Input to HEC-FDA requires stage-elevation information for each cross section for the following return period flows: 1:2, 1:5, 1:10. 1:25, 1:50, 1:100, 1:250 and 1:500 year. For most models in the UTRCA watershed, these flows have been previously calculated, with the exception of the 1:500. In this case, the flow was estimated by extrapolating the known flows on a logarithmic scale

18 VI.5 Economics as: The economics section of the HEC-FDA model involves entering information such Study Damage Categories Structure Occupancy Types Structure inventory information Study damage categories are straightforward for this project, using residential, commercial, public and industrial. Agricultural damage could be included as an additional category, but the present version of HEC-FDA is not oriented towards these calculations, so an estimate of agricultural flood damage will need to be done outside this program. Structure occupancy type refers to the use of the structure (ie type of residence or commercial business) and these are taken directly from the generic depth damage tables presented in the MNR Flood Damage Estimation Guide. Structure inventory simply allows the data entry of all of the information obtained in Section III above. VI.5.1 Application of Depth Damage Tables As the most current depth damage tables available are those compiled by the MNR in their Flood Estimation Guide, these are the ones that were used for this study. The MNR tables are presented in a depth vs. direct damage (in 1984 $) from 8 feet below grade to 8 feet above grade, in other words 0 feet is meant to be the grade elevation of the structure. HEC-FDA has different input options for depth damage curves, either allowing each specific structure to have its own direct damage curve, which would be in the same format as the MNR data is presented, or in the form of a more general curve

19 meant to address a whole subclass of structures (i.e. one storey with basement, one storey without basement etc ). The second option, using general curves is more suitable, mainly as the curves would not change from structure to structure. However, the data entry for this method requires tables in a depth vs. % damage format, along with the entry of a total value for each structure. Data was thus translated into this format, assuming that the 8 foot above ground damage was 100% of possible damage. We should also note that HEC-FDA has much flexibility in terms of depth damage curve entry, giving the user the ability to enter one curve for structure damage, one for content damage and one for other damage. The curves produced by the MNR however include structure and content damage, and the Flood Damage Estimation manual suggests other damages be estimated as a percent of direct flood damage, tabulated from various sources, and generally between 10% and 40%. The Glengowan Hydrology and Flood Damage Study suggests using 15%, 20% and 20% of direct damage costs for residential, commercial and industrial structures respectively. This study will continue to use these suggested values

20 VII. STUDY DAMAGE REACHES. The following figures illustrate the areas used as damage reaches for each of the damage centres in the Upper Thames basin, as well as provides an overview of the extent of potential flood damage

21 VII.1 Mitchell

22 VII.2 St. Marys

23 VII.3 London

24 VII.4 Ingersoll

25 VIII. HEC-FDA RESULTS VIII.1 Flow Damage Curves Damage results from the HEC-FDA package are presented in terms of stagedamage for each damage centre in the watershed. A more useful presentation of this data for the purposes of this study is in terms of Flow-Damage rather than stage damage as this integrates better with the output from the Hydrologic model (HEC-HMS) used for estimating flows under various climate change scenarios. Plots of all Flow- Damage Curves are estimated for all reaches, for all damage categories, for each damage centre presented in sections to Note also that not all damage reaches reported damage when the FDA model was executed, and thus output for these reaches are not plotted

26 VIII.1.1 Mitchell

27 - 27 -

28 VIII.1.2 St Marys

29 - 29 -

30 VIII.1.3 London

31 - 31 -

32 - 32 -

33 - 33 -

34 - 34 -

35 - 35 -

36 - 36 -

37 VIII.1.4 Ingersoll

38 VIII.2 Aggregated Flow Damage Curves The Flow-Damage information presented above for each individual damage centre provides useful information for the Conservation Authority and its member municipalities in the event of a major flood. However, for the purposes of the present study, the flow

39 damage estimates are aggregated to stream gauge sites, which correspond to subwatersheds delineated and modeled by the HEC-HMS model used in this study, wherever possible. VIII.2.1 Mitchell Mitchell damage is aggregated to flows at the Mitchell stream gauge, with the assumption that both the Whirl Creek and North Thames river watersheds are contributing flow to the gauge. From previous work done in modelling flood lines and estimating the hydrology in these two watersheds the following relationships are derived: Q North Branch = Q Mitchell x 0.57 Q Whirl Cr. = Q mitchell x 0.49 VIII.2.2 St Marys

40 St Marys damage mostly occurs below the confluence with Trout Creek, and thus it is simple to aggregate all damages via flow damage relationship to the St. Marys Stream Gauge. Also, we can summarise damages on the Trout Cr. damage reach but aggregating it with the discharge from Wildwood dam. VIII.2.3 London London results can be aggregated to stream gauges as follows. VIII North Branch Discharge from Fanshawe dam can be used to estimate damage from the dam to the confluence with Medway River (to damage reach 16c). Release from Fanshawe Dam plus value at Medway River gauge can be used to estimate damages from the confluence to the forks

41 VIII South Branch As the south branch does not have any significant tributary input within London (with the exception of the ungauged Pottersburg Cr., which generally peaks before the South Branch) all damage for this branch can be aggregated to the Ealing stream gauge

42 VIII Main Branch Flood damages on the main branch of the river between the forks and the most downstream London damage reach (reach 1) are aggregated to the Byron Stream Gauge. VIII.2.4 Ingersoll For the purposes of the present study, only flood damages caused by flooding of the Thames River proper can be considered as the watershed hydrology model does not have enough detail in its catchment delineation to consider flooding on the 5 tributaries which enter the Thames River in Ingersoll. Flood damages for Ingersoll thus are aggregated to the Ingersoll Stream Gauge

43 - 43 -

44 IX. CONCLUSIONS A detailed flood damage estimation was completed for this report as part of the CFCFA study Assessment of Water Resources Risk and Vulnerability to Changing Climactic Conditions. The detailed results produced by the Damage estimation model HEC-FDA were then translated from elevation-damage based output, to a flow-damage based output for each damage centre in the Upper Thames watershed. This information was then further aggregated to created flow damage relationships at each stream gauge within each major damage center, which coincide with subwatershed outlets delineated in thy hydrological model created for this study. These tables will be incorporated into the System Dynamics model concurrently being developed by the University of Western Ontario members of the team working on the present study to evaluate the effects of climate change on Flood Damages in the Upper Thames River Watershed

45 X. REFERENCES [1] Conestoga-Rovers & Associates, Flood Protection Engineering, Town of St. Marys. [2] Cumming Cockburn Limited, Broughdale Dyke Study. [3] Cumming Cockburn Limited, Ingersoll Floodway Study. [4] Kontzamanis Graumann Smith Macmillan Inc., Red River Basin. Stage- Damage Curves Update and Preparation of Flood Damage Maps Draft Final Report. [5] Marshall Macklin Monaghan, Background Report to the Glengowan Environmental Assessment. Report No. 9 Hydrological and Flood Damage Study. [6] Marshall Macklin Monaghan, Flood Damage Reduction Study Town of St. Marys. [7] Ontario Department of Planning and Development, Upper Thames Valley Conservation Report. [8] Ontario Ministry of Natural Resources, Flood Damage Estimation Guide. [9] Ontario Ministry of Natural Resources, Evaluation of Water Resources Management Strategies and Flood Damages. [10] Paragon Engineering Limited, Development of Flood Depth Damage- Curves for residential homes in Ontario. [11] Upper Thames River Conservation Authority, Town of Mitchell Floodline Study. [12] Upper Thames River Conservation Authority, Hydraulic Computer Model for the City of London (HEC-2 Update 1985). [13] Upper Thames River Conservation Authority, Calculated Water Surface Elevations for Floodplain Management within the City of London. [14] Upper Thames River Conservation Authority, Regional and 1:100 Year Floodlines on the North Thames River in St. Marys and Trout Creek from St. Marys to Wildwood Dam. [15] Upper Thames River Conservation Authority, Damage Assessment Management Program. North Thames River and Whirl Creek in Mitchell. [16]. US Army Corps of Engineers, Engineering and Design. Risk Based

46 Analysis for Flood Damage Reduction Studies. EM [17] US Army Corps of Engineers, HEC-FDA: Flood Damage Reduction Analysis User s Manual, Version